1. Field of the Invention
The present invention relates to a liquid crystal display device which is constituted by a two-layered STN (super twisted nematic) liquid crystal device to compensate the colored display obtained by a single STN-LCD to have a black and white display.
2. Description of the Related Art
Recently, a dot matrix liquid crystal display device for displaying a television image or the like tends to have a significantly large number of display pixels to ensure a larger active area and higher resolution. Accordingly, the liquid crystal display device is demanded to be multiplex-driven at high duty.
When a TN type liquid crystal display device which has been widely used is multiplex-driven, its operation margin decreases and the display contrast decreases, thus impairing the view angle characteristic. In this respect, the TN type liquid crystal display device is not suitable for multiplex-driving.
To improve the multiplex-drive characteristic, STN type liquid crystal display devices (STN-LCD) have been developed which have liquid crystal molecules twisted at a greater twist angle (e.g., 180.degree. to 270.degree.) than that of the ordinary TN type liquid crystal display device.
This STN-LCD is of a double refraction effect type which has its polarizing plate arranged in such a way that the direction of the axis of polarization (transmitting axis or absorption axis) forms an angle of 35.degree. to 50.degree. to the direction of orientation of the liquid crystal molecules adjacent to that surface of the substrate on the side of the polarizing plate. A linear polarized light coming through a polarizing plate on the light incident side becomes an elliptically polarized light due to the double refraction effect (birefringence) of the liquid crystal layer. That part of the elliptically polarized light which passes through the polarizing plate on the light-leaving side causes an image to be displayed. This STN-LCD utilizes the double refraction effect of the liquid crystal layer, which has a different refractive-index anisotropy for each wavelength. This property of the liquid crystal layer increases the transmittance of light in a specific wavelength range to thereby provide a colored display.
To eliminate such coloring of the display screen caused by the STN-LCD, a two-layered STN-LCD has been proposed in which a color-compensating liquid crystal cell is provided on the light incident side or light-leaving side to eliminate the coloring effect.
This two-layered STN-LCD comprises a display liquid crystal cell, a color-compensating liquid crystal cell and a pair of polarizing plates. The display liquid crystal cell has a display electrode and has liquid crystal molecules twisted at an angle of 180.degree. to 270.degree.. The color-compensating liquid crystal cell comprises a layer of liquid crystal molecules twisted at the same angle as those of the display liquid crystal cell but in the opposite direction, and a STN type liquid crystal cell oriented in such a way that the orientation directions of its liquid crystal molecules adjacent to the adjoining substrate surfaces of the display liquid crystal cell and the color-compensating liquid crystal cell are normal to each other. The polarizing plate pair are disposed in such a way that they sandwich these layered two cells and that the direction of the transmitting axis of one of the polarizing plates crosses at an angle of 35.degree. to 50.degree. to the orientation direction of the liquid crystal molecules on the substrate surface of the liquid crystal cell adjacent to this polarizing plate on the side thereof, and the direction of the transmitting axis of the other polarizing plate is substantially normal to that of the transmitting axis of the former polarizing plate.
This arrangement permits the phase difference or retardation between the ordinary light and extraordinary light for each wavelength of light passing one of the liquid crystal cells to be canceled out by the other liquid crystal cell due to dispersion of rotatory polarization of the liquid crystal layers of both liquid crystal cells. Therefore, coloring of the display screen can be substantially eliminated.
In order to cancel out the retardation between the ordinary light and extraordinary light for each wavelength of light passing one liquid crystal cell, the phase difference between the ordinary light and extraordinary light for each wavelength of light passing the other liquid crystal cell should be made coincident with that of the former liquid crystal cell. If both liquid crystal cells are produced in the same process, the elements for changing the retardation such as the thickness of a liquid crystal layer (cell gap) becomes stable so that the produced liquid crystal cells can have uniform retardation. Because of these reasons, conventionally both liquid crystal cells are made of the same liquid crystal material, are subjected to the same aligning treatment and are given with the same cell gap.
When this two-layered STN-LCD is multiplex-driven, however, unselected elements and half-selected elements of a plurality of pixels appear to be colored. That is, the aligning area of the pixels appears as a colored background so that a B/W display cannot be provided.
Further, since the double refraction of the liquid crystal material significantly changes by temperature, the amount of compensation varies with a change in temperature of the liquid crystal display, thus changing the color of the display screen.
Furthermore, due to the temperature-dependency of the liquid crystal material, the value of the optical anisotropy An of the liquid crystal at a high temperature side becomes smaller, and .DELTA.n.multidot.d also becomes smaller, thus reducing the transmittance. In addition, since the viscosity of the liquid crystal becomes high at a low temperature side, the response is significantly reduced.